1. Field of the Invention
The present invention relates to a process for the production of a ferromagnetic metal powder. More particularly, the present invention relates to a process for producing a ferromagnetic metal powder for a magnetic recording member suitable for high density recording.
2. Description of the Prior Art
Ferromagnetic powders hitherto used in magnetic recording members are maghemite (.gamma.-Fe.sub.2 O.sub.3), Co-doped maghemite, magnetite (Fe.sub.3 O.sub.4), Co-doped magnetite, a Berthollide compound of maghemite and magnetite (FeO.sub.X, 1.33 &lt; x &lt;1.50), a Co-doped Berthollide compound of maghemite and magnetite, chromium dioxide (CrO.sub.2), and the like. These ferromagnetic powders, however, had insufficient magnetic properties such as coercive force (Hc), maximum residual magnetic flux density (Br), and the like when used in the so-called high density recording. They are, therefore, not suitable for magnetic recording in which signals of a short wave length about 10 .mu.m or less) are recorded.
Recently, investigations on ferromagnetic powders having properties suitable for high density recording have been widely conducted. One example of these ferromagnetic powders which have been investigated is a ferromagnetic metal powder.
The following methods of producing ferromagnetic metal powders are known.
1. A method comprising heat-decomposing an organic acid salt of a metal capable of forming a ferromagnetic material, for example, an oxalate, and then reducing with a reducing gas. This method is described in, for example, Japanese patent publication Nos. 11412/1961, 22230/1961, 14809/1963, 3807/1964, 8026/1965, 8027/1965, 15167/1965, 12096/1966, 24032/1967, 3221/1968, 22394/1968, 29268/1968, 4471/1969, 27942/1969, 38755/1971, 38417/1972, 41158/1972, 29280/1973, and U. S. Pats. 3,186,829, and 3,190,748.
2. A method comprising reducing a needle-like iron oxide derived from an oxyhydroxide of iron, or a needle-like oxyhydroxide of iron, or a substance containing metals other than iron as well as the above oxyhydroxide. This method is described in, for example, Japanese patent publication Nos. 3862/1960, 11520/1962, 20335/1964, 20939/1964, 24833/1971, 29706/1972, 39477/1972, 24952/1973, Japanese patent application OPI Nos. 5057/1971, 7153/1971, 79153/1973, 82395/1973, and U.S. Pat. Nos. 3,598,568, 3,607,220, and 3,702,270.
3. A method comprising evaporating a ferromagnetic metal in a low pressure inert gas. This method is described in, for example, Japanese patent publication Nos. 25620/1971, 4131/1972, 27718/1972, and Japanese patent application (OPI) Nos. 25662/1973 through 25665/1973, 31166/1973, 55400/1973, and 81092/1973.
4. A method comprising heat-decomposing a metal carbonyl compound. This method is described in, for example, Japanese patent publication Nos. 1004/1964, 3415/1965, 16868/1970, and U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007, and 3,288,882.
5. A method comprising electrodepositing a ferromagnetic metal powder using a mercury cathode, and then separating the metal powder from mercury. This method is described in, for example, Japanese patent publication Nos. 12910/1960, 3860/1961, 5513/1961, 787/1964, 15525/1964, 8123/1965, and U.S. Pat. Nos. 3,198,177, 3,156,650, and 3,262,812.
6. A method comprising reducing a solution containing a metal salt having ferromagnetic properties by adding a reducing agent thereto. This method is described in, for example, Japanese patent publication Nos. 20520/1963, 26555/1963, 20116/1968, 9869/1970, 14934/1970, 7820/1972, 16052/1972, 41718/1972, Japanese patent application (OPI) Nos. 1363/1972, 42252/1972, 42253/1972, 44194/1973, 79754/1973, 82396/1973, and U.S. Pat. Nos. 3,607,218, 3,756,866, 3,206,338, 3,494,760, 3,535,104, 3,567,525, 3,661,566, 3,663,318, 3,669,643, 3,672,867, and 3,726,664.
The present invention is concerned with the solution reduction process as shown in the above method (6). In particular, the present invention is concerned with a post-treatment of ferromagnetic metal powders produced by the method in which metal salts containing ferromagnetic metals are reduced in solutions thereof, i.e., the above described method (6).
In general, the ferromagnetic metal powder produced by method (6), after separation of the reaction mother liquor, and removal of anions such as unreacted metal ions, sulfate ion (SO.sub.4.sup.2-), chloride ion (Cl.sup.-), and the like by sufficient water-washing, is treated by replacing the solvent or by drying in an inert gas, i.e., an non-oxidizing atmosphere. The thus prepared ferromagnetic metal powder, however, has poor magnetic stability and, in particular, is easily affected by humidity, i.e., it is gradually oxidized due to the water vapor in the air. Thus, the magnetic properties of these powders containing iron as the major component often are lost. Moreover, if this oxidation phenomenon takes place during a post-treatment after the reduction, the high maximum magnetic flux density (Bm), one of the excellent characteristics of the ferromagnetic metal powder, tends to be lost.
Various methods have been proposed in the past to remove these problems. For example, a method comprising adding reducing non-magnetic elements to the reaction bath to provide resistance to oxidation is described in Japanese patent publication Nos. 20520/1963, 7820/1972, Japanese patent application (OPI) Nos. 78896/1975 (corresponding to U.S. patent application Ser. No. 524,858, filed Nov. 18, 1974 now Pat. No. 4,007,072) and 78897/1975 (corresponding to U.S. patent application Ser. No. 524,860, filed Nov. 18, 1974), U.S. Pat. Nos. 3,535,104, 3,669,643, and 3,672,867, etc. Another method comprising adding an organic compound an additive to the reaction solution is described in Japanese patent publication Nos. 20116/1968, 14934/1970, 7820/1972, 7820/1972, 42253/1973, 44194/1972, 79754/1973, 82396/1973, and U.S. Pat. No. 3,607,218, etc.
In particular, a method comprising carrying out the reaction in the presence of higher aliphatic acids as described in Japanese patent publication No. 20116/1968, a method comprising carrying out the reaction in the presence of compounds containing sulfone groups or an aromatic ring as described in Japanese patent publication No. 14934/1970, and a method comprising carrying out the reaction in the presence of surface active agents as described in Japanese patent publication No. 7820/1972, are well known.
According to these methods, the reaction is carried out after adding surface active agents and so on to the reaction bath. However, it has been found that the addition of the surface active agents and the like in amounts sufficient to increase the maximum magnetic flux density (Bm) and resistance to oxidation only produces ferromagnetic metals which have poor coercive forces (Hc) and squareness ratios (Br/Bm). In other words, where the surface active agents and the like are added to the reaction bath to carry out the reaction, it is impossible to obtain ferromagnetic metal powders having high maximum magnetic flux density (Bm) and high resistance to oxidation without deteriorating their magnetic properties, i.e., the coercive force and the squareness ratio.
As a result of various investigations on methods of preventing oxidation from occurring during the post-treatment after the reduction in producing ferromagnetic metal powders using a solution reduction process, it has been found that oxidation during water-washing is the most significant. Thus, methods of washing with an aqueous solution containing saccharin, an aqueous solution containing a reducing material, and the like, have been proposed in Japanese patent application (OPI) Nos. 18345/1975, 19667/1975, 41097/1975 (corresponding to U.S. patent application Ser. No. 497,794 now U.S. Pat. No. 3,966,510 , filed on Aug. 15, 1974), 41756/1975 (corresponding to U.S. patent application Ser. No. 498,338, filed on Aug. 19, 1974), 104164/1975, 104397/1975 corresponding to U.S. patent application Ser. No. 598,098 filed on July 21, 1975 now U.S. Pat. No. 4,020,236 ), and 106198/1975, and Japanese patent application No. 9698/197 (corresponding to U.S. patent application No. 600,245 filed on July 24, 1975).
The present invention provides a more improved method of producing ferromagnetic metal powders.